why do so many discussions turn contentious?

Well, it’s the first of Seven Deadly Sins. The Vainglory. Some people just like to feel themselves gurus, they collect enormous amounts of information, almost never implement this information practically or even theoretically but they think they are gurus and they know all answers. When you try to question their opinion or deny their teaching, discussion usually goes nasty way. 

Our lack of social evolution also tied much to our physical natures, testosterone and alpha wars.

I still love this forum

It’s because people that exist primarily online have almost zero consequences for their cowardly bullying  responses. They enjoy their “ virtual” power and most likely are miserable souls. They try to politicize anything. I bet most of them in person do not act that way. Jekyl in person and Hyde online. All of my hobbies with online forums have become like this in the last decade or so. Years ago when I first started seeing “ what is the best” forums I knew we were in trouble. Somehow winning has infiltrated everything in our culture.

I have always been impressed by the amount of knowledge and passion for audio and music expressed in these forums.  I like how people are willing to share their experiences with others.  This is true when I first joined (2000?) and it's still true today.  I think the OP's question is a little Pollyanna-ish in that there's always been heated discussions, sometimes respectful and courteous, but frequently childish and mean spirited.  It's never been perfect.

What encourages me about this thread is the number of people who have contributed who have low posting counts.  Is it possible that there's a vast silent majority of forum users?  I have 4,000 some odd posts, but it's taken me twenty years to reach that total.  My post are always highly intelligent, insightful, technically astute, historically accurate and built on a foundation of unassailable logic, but even I wonder if I really had 4,000+ things to say about audio and music.  Yet I've also noticed that some user(s) have multiples of my total posts, but have only been posting for two years.  Is it actually possible for someone to have 15,000 things to say that are worth saying within two years?  The obvious answer is no.  The easiest solution for dealing with people who really don't have anything worthy to contribute is to just ignore them.  You see their names multiple times in a thread -- don't read the thread.  In the long run you'll be happier.